Today, PVC based rigid foam polymer materials are being widely used, mainly as core material in sandwich structures in the naval or aeronautic sector, or as thermal/acoustic insulators in the building sector. In a sandwich structure the core separates two structurally more rigid materials, such as fibre reinforced plastics (FRP), metal or the like. Such sandwich structures have many advantages compared to more traditional single layer structures, such as lower weight, insulation properties etc. Whilst other rigid foam polymer materials, such as foamed polyurethane etc. can be produced using streamlined continuous extrusion methods, the production of PVC based rigid foam polymer materials involves moulding of discrete partially expanded bodies (hereafter referred to as embryo bodies) under high pressure in a press. The embryo bodies are subsequently subjected to a chemical-physical treatment to obtain the rigid foam polymer material.
More in detail, the production process of a PVC based rigid foam polymer material initially involves formation of a plastisol paste consisting of a mixture of powders (PVC and other compounds) and liquid substances (in particular isocyanates). The paste filled in a closed mould cavity and is subjected to a heating and subsequent cooling process under high pressure resulting in a partially expanded embryo body. The embryo body is then further expanded through an additional heat treatment in water and/or a steam oven. The formation of the final rigid foamed material is a result of a hydrolysis reaction of the isocyanate groups present in the material, with subsequent build up of a polymer which crosslink the chemical structure.
At present, the methods for the production of embryo bodies involves filling the each mould with an excess amount of paste with respect to the polymer content in the finished product. The excess amount is then allowed to leak out from the mold during the molding process. The moulding process comprises heating the plastisol in a closed mould, whereby a high pressure is created by the thermal expansion of the plastisol and the activation of the blowing agent dissolved therein. During this expansion step, the excess amount is allowed to leak out. The plastisol is kept at elevated temperature a predetermined time to allow the plastisol to gelatinize, where after the mould cavity is cooled to a temperature that is low enough to remove the embryo body from the mould. The excess amount is approximately equal to about 8%, in terms of weight, of the product leaving the mould.
The excess paste emerges from the top edge of the mould. Consequently there is a non-recoverable wastage of material, since the PVC gelatinizes and some of the blowing agent substances deteriorate at the high temperature.
U.S. Pat. No. 6,352,421, Olivier Giacoma, filed 2000 Feb. 15, solves the problem of escape of paste from the mould during the heating step by providing a secondary mould compartment into which the excess of paste is fed during the heating step, and from which a small part of the paste is allowed to escape into a perimetral waste-collecting groove. According to the disclosed method, paste is top-filled in the primary mould compartment, during heating the paste expands about 8% and excessive paste is fed to the secondary compartment by connecting grooves. The secondary compartment has a volume that is slightly less than 8% of the primary compartment. Hence, the amount of waste paste is reduced to about 8% of the volume of the secondary compartment, which is approximately equal to 0.64% of the volume of the primary compartment.
U.S. Pat. No. 2,768,407, Lindemann, Filed Dec. 5, 1950, relates to production of closed cell cellular bodies from thermo-plastic masses. It is stated that a problem in the prior art is that it is in practice impossible to keep a mould filled with a mass containing an expansion agent completely sealed during the heating stage. A solution to this problem is proposed: by, after the gases have been dissolved under pressure in the mass and the mass has gelatinised completely, expanding the volume of the mould by ⅕ to ⅖ of the original volume. Use of a mould with a moveable die is proposed, and it is stated that it is necessary to apply a high pressure e.g. 150-300 atm (bar) to slow down the decomposition of the expansion agent and to cause the gas to dissolve. It is also stated that suitable thermo-plastics include polyvinyl-chloride.